Sodium citrate (Na₃Cit)-assisted hydrothermal synthesis and characterization of twinned hemisphere shaped La₂(MoO₄)₃:Eu³⁺ phosphors

Abstract

La₂(MoO₄)₃:Eu³⁺phosphors with a uniform twinned hemisphere morphology have been successfully synthesized by a sodium citrate (Na₃Cit) mediated hydrothermal approach. The crystal structure has been determined by X-ray powder diffraction (XRD) and the microstructures were characterized by scanning electron microscopy (SEM). The controlled experiments indicate the reaction temperature and reaction time are responsible for shape determination of the La₂(MoO₄)₃:Eu³⁺ products. The possible formation mechanism for these particles is presented by increasing the reaction time. The emission spectra of the samples at different reaction times exhibited intense emission bands at different wavelengths. Under UV light excitation, La₂(MoO₄)₃:Eu³⁺ emits bright red luminescence. This property shows that these materials have potential applications in lighting and display fields.

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1. Introduction

Recently, the synthesis of inorganic micro/nanocrystal materials with well-defined morphologies and accurately tunable sizes has attracted much attention allowing for their applications in optics, electrics, catalytic, magnetic, and optoelectronics.^1–5 Especially, intensive research efforts have been focused on luminescent materials. Among them, WLED are expected to continue to grow very fast in all applications due to their fascinating merits of energy savings, higher color rendering index (CRI), higher brightness, longer life time and environmental friendliness compared with conventional incandescent and fluorescence lamps.^6–8 The most common way to obtain white light is to employ the yellow-emitting YAG:Ce³⁺ phosphor and blue InGaN (370–460 nm) chips. Unfortunately, the CRI value is low due to their poor emission in the red region.^9,10 Thus, it is meaningful to develop some novel red-emitting phosphors with high luminous efficiency, small size, controllable morphology, and environmentally friendly characteristics for WLED via chemical approaches.

Molybdates can exist in different crystal phases composed of the same or different valences of molybdenum element. In addition, molybdates are suitable host materials for rare earth ions and display some excellent performance.^7,24–27 Rare earth ion doped molybdates are interesting because of their physical properties and potential applications in modern lighting and display fields.^{11–17,22,23} Among these RE ions, the Eu³⁺ ion is an important activator that can show characteristic red emission due to the transitions of ⁵D₀ → ⁷F₂.^18–21 Previously, L. Krishna Bharat et al. have reported the spindle shaped La₂(MoO₄)₃:Eu³⁺ phosphors by a polyol mediated solvothermal synthesis method.⁶ Lin et al. also have reported the La₂(MoO₄)₃ with various self-assembled three-dimensional hierarchical structures synthesized via a hydrothermal method in controlling the EDTA concentration and the pH of solution.²⁸ Chen et al. have reported the monodisperse La₂(MoO₄)₃:Yb,Tm microarchitectures with uniform waxberry-like morphology constructed in large scale successfully by a facile surfactant-assisted hydrothermal route, using sodium lauryl sulfate (SLS) as a structure-directing agent.²⁹ Watching over those works, we present the synthesis of twinned hemispheres La₂(MoO₄)₃:Eu³⁺ phosphors by a sodium citrate (Na₃Cit) mediated hydrothermal approach. Among these various methods, hydrothermal method is one of the significant techniques, which can meet the demand of modern chemistry for the synthesis of the materials with controllable size, morphology and crystal structure. Especially by adding different surfactants, novel structure materials with controlled morphology can be obtained, which thanks to their efficient self-assembly properties in aqueous solution. Up to now, hydrothermal synthesis theory and surfactant action theory are still not perfect and remain longstanding challenge.

In this paper, we report a sodium citrate (Na₃Cit) mediated hydrothermal approach for the preparation of monodisperse La₂(MoO₄)₃:Eu³⁺ micro-architectures with uniform twinned hemispheres morphology. Finally, we preliminarily investigate the structure, morphology and optical properties of as-synthesized products in detail.

2. Experimental

2.1 Materials

All chemical reagents were analytical grade and used as received. The rare earth oxide Eu₂O₃ (Chang Chun Applied Chemistry Science and Technology Ltd., China), La₂O₃ (Tianjin Guangfu Fine Chemical Research Institute, China), sodium molybdate (Na₂MoO₄·2H₂O) (Tianjin Chemical Reagent Ltd., China), sodium citrate (Na₃Cit) (Shanghai Chemical Reagent Ltd., China), nitric acid (HNO₃) (Beijing Chemical Reagent Ltd., China). All aqueous solutions were prepared with deionized water.

2.2 Preparation

In a typical experimental process, 0.05 mmol Eu₂O₃ and 0.95 mmol La₂O₃ were dissolved in diluted HNO₃. And then we removed the extra HNO₃ through heating and stirring, respectively, resulting in the formation of Eu(NO₃)₃ and La(NO₃)₃ crystal powders. Afterward, Eu(NO₃)₃ crystal powders, La(NO₃)₃ crystal powders, 3 mmol Na₂MoO₄·2H₂O and 1 mmol Na₃Cit were dissolved in 25 mL deionized water with magnetic stirring for 30 minutes. The final solution was transferred into a 50 mL Teflon-lined autoclave and heated at 180 °C for 12 h. After the autoclave was naturally cooled down to room temperature, the products were collected by centrifugation and washed several times with absolute ethanol and deionized water. Finally, the as-obtained products were dried in vacuum at 80 °C for 8 h.

2.3 Characterization

The samples were characterized by the X-ray diffractometer (XRD) on a Rigaku-Dmax 2500 diffractometer (Japan) with Cu-Kα radiation (λ = 1.5406 Å) at a scanning rate of 6° min⁻¹ in a 2θ range from 10–80°. The morphology of the samples were inspected using a field emission scanning electron microscope (FE-SEM, JSF-6700) and a JEOL 2010 transmission electron microscope (TEM) equipped with a field emission gun at a voltage of 200 kV for FE-SEM and TEM images, respectively. The Fourier transform-infrared (FT-IR) spectrum was measured via PerKing-Klmer 580B infrared spectrophotometer with the KBr pellet technique. The photoluminescence (PL) excitation and emission spectra were obtained using a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Phase identification and morphology

The composition and phase purity of the products were characterized by XRD. The XRD patterns of the as-prepared La₂(MoO₄)₃:Eu³⁺ products obtained at 180° for 12 h with and without 1 mmol Na₃Cit via hydrothermal process are shown in Fig. 1a and b. The diffraction peaks of the two samples can be readily indexed to the pure tetragonal phase of La₂(MoO₄)₃ with the space group of I4₁/a, agreeing well with the literature values (JCPDS no. 45-0407). All peaks located at 2θ values of 15–65° mostly correspond to the characteristic diffractions and no other peaks were detected, revealing the high purity of the as-obtained products. It is very important for phosphors because higher crystalline generally means stronger luminescence. In general, the nanocrystallite size can be estimated from the Scherrer formula: D_hkl = Kλ/(β cos θ), where λ is the X-ray wavelength (0.15405 nm), β is the full-width at half maximum, θ is the diffraction angle, K is a constant (0.89) and D_hkl means the size along the (hkl) direction.⁶ After the addition of sodium citrate (Na₃Cit), the XRD diffraction peak becomes wider (Fig. 1a), indicating that the size of the as-prepared becomes smaller.

Fig. 1 The XRD patterns of La₂(MoO₄)₃:Eu³⁺powders synthesized at 180 °C for 12 h with (a) and without 1 mmol Na₃Cit (b), as well as the standard data for La₂(MoO₄)₃ (JCPDS 45-0407).

The morphology of the La₂(MoO₄)₃:Eu³⁺ particles were studied by field emission scanning electron microscopy (FE-SEM). In the present work, it was found that Na₃Cit introduced to the reaction system had a crucial effect on the morphology and size distribution of the final products. Thus, we performed control experiment at different dosage of Na₃Cit at the reaction temperature of 180 °C for 12 h, with the other parameters kept constant. With 1 mmol Na₃Cit, the low-magnification SEM image shown in Fig. 2a clearly reveals that there exists a great deal of well-dispersed and uniform microspheres approximately 1 μm in diameter. As shown by the high magnification images in Fig. 2b, the as-synthesized samples were uniform microspheres composed of twinned hemispheres with an average diameter of about 1 μm and there was a clear joint boundary between the twinned hemispheres. Fig. 2c shows the surface of the spherical structure is not smooth but composed of numerous nanoparticles. Fig. 2d displays a great deal of irregular twinned hemispheres and some sphere with agglomeration were produced in the absence of Na₃Cit.

Fig. 2 FESEM image of La₂(MoO₄)₃:Eu³⁺phosphors obtained in the presence of 1 mmol Na₃Cit (a–c) and without (d) at 180 °C for 12 h.

The presence of functional groups in the compound was evaluated by the FT-IR spectra at 180°, 12 h with (Fig. 3a) and without (Fig. 3b) 1 mmol Na₃Cit, as shown in Fig. 3. The broad peak centered at 3391 cm⁻¹ is ascribed to the stretching vibrations of the hydroxyl group (O–H). The bands at 1575 cm⁻¹ and 1396 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of C=O, respectively, suggesting formation of La³⁺–citrate complexation. The existence of Na₃Cit in the compound is confirmed by the appearance of absorption peaks at 1396 and 1575 cm⁻¹. The multiple splitting bands in the region below 912 cm⁻¹ are assigned to the stretching vibrations of the MoO₄²⁻ cluster. A strong absorption peak in the region of 804–716 cm⁻¹ corresponds to the anti-symmetric stretching vibrations of Mo–O bond.^30,31

Fig. 3 FT-IR spectra of La₂(MoO₄)₃:Eu³⁺samples fabricated at 180 °C for 12 h with (a) and without 1 mmol Na₃Cit (b).

The TEM images (Fig. 4) also confirm the morphology of the sample corresponding well with the SEM (Fig. 2) results. Twinned hemispheres morphology of the La₂(MoO₄)₃:Eu³⁺nanoparticle was prepared by Na₃Cit (1 mmol) mediated hydrothermal reaction method at 180 °C for 12 h, as shown in Fig. 4a–c. It is obvious that these nanospheres are highly uniform and homogeneously dispersed, which is beneficial to improve the luminescence property. A TEM image (Fig. 4d) of the sample without Na₃Cit shows that the products are composed of irregular sphere shaped nanoparticles. The above discussion indicates that the Na₃Cit are responsible for shape determination of the La₂(MoO₄)₃:Eu³⁺ products. In the early stage of the reaction, uniform La³⁺–Cit³⁻ intermediate solid spheres are formed, as confirmed by FTIR, SEM analysis. The as-obtained intermediate is employed as both physical and chemical template, which not only cast the morphology of the product but also affords a reactant source for an interfacial reaction.

Fig. 4 TEM image of La₂(MoO₄)₃:Eu³⁺phosphors obtained in the presence of 1 mmol Na₃Cit (a–c) and without (d) at 180 °C for 12 h.

3.2 Factors influencing the formation of La₂(MoO₄)₃ phosphors

It is found that the reaction temperature plays an important role in determining the morphology and size distribution of the final product. The corresponding XRD patterns obtained at different reaction temperatures were shown in Fig. 5. The diffraction peaks of the different samples prepared from the precursor suspensions at the temperature of 120 °C, 140 °C, 160 °C, 180 °C for 12 h, keeping other experimental conditions unchanged, can be readily indexed to the pure tetragonal phase of La₂(MoO₄)₃. By varying the reaction temperature, we can find that the morphologies of the as-synthesized products change gradually from peanut-like to twinned hemispheres. The corresponding SEM images have been presented in the Fig. 6a–c. At 120 °C, the dominant products are peanut-like particles with the full length about 3 μm and the width about 1 μm, respectively (Fig. 6a). From the magnified image shown in the Fig. 6b, these microspheres with coarse surfaces were entirely constructed by nanoparticles. When the reaction temperature was increased to 180 °C, we can clearly see that each particle was divided into two hemispheres with a joint boundary between them. Each hemisphere had an average diameter of approximately 1 μm, (Fig. 6c).

Fig. 5 XRD patterns of La₂(MoO₄)₃:Eu³⁺ products obtained at (a) 120 °C, (b) 140 °C, (c) 160 °C, (d) 180 °C for 12 h.

Fig. 6 FESEM images of the products prepared at (a and b) 120 °C, (c) 180 °C for 12 h.

To substantially understand the formation mechanism of these unique twinned hemispheres micro-architectures, a series of time-dependent experiments were carried out. Fig. 7 shows FE-SEM images of the samples obtained by using Na₃Cit as the chelating reagent in different reaction time at 180 °C under hydrothermal conditions. As shown in Fig. 7a, in the initial 3 h of the reaction, the products were composed of irregular peanut-like particles. As the reaction time was allowed to proceed to 6 h, the products are consisted of uniform peanut-like micro-architecture with a high yield, and transformed into well crystallized (Fig. 7b). When the reaction time was extended to 12 h, the morphology of the particles was gradually changed and formed twinned hemispheres, as can be seen in Fig. 7c. Finally, when the reaction time increases to 24 h, the morphology of the product was not much different from the microstructures gained for 12 h, but the diameter of hemispheres become larger, as shown in Fig. 7d. However, a closer observation (Fig. 7d) indicates that there are some small spheres aggregate in the product.

Fig. 7 FESEM images of the samples synthesized at 180 °C for different reaction stages: (a) 3 h, (b) 6 h, (c) 12 h, (d) 24 h.

On the basis of time-dependent morphological evolution, the growth mechanism of the La₂(MoO₄)₃:Eu³⁺ twinned hemispheres constructed can be illustrated as follows (Scheme 1). In the first stage, the introduction of molybdate into the La³⁺–Cit³⁻ complex led to the formation of La₂(MoO₄)₃ nanoparticles by the competition of MoO₄²⁻ for La³⁺ with Cit³⁻. The Na₃Cit on the surface of La₂(MoO₄)₃ nuclei probably have the function of kinetically controlling the growth of the fresh nanoparticles through the interaction of the adsorption and desorption processes. In the second stage, the larger microspheres mediated by hydrogen bonds were formed via the self-assembly process of the primary nanoparticles in order to minimize the interfacial energy. The Na₃Cit here can be considered as a “cohesive agent”. The products experienced self-aggregation accompanying with Ostwald ripening process and formed uniform microspheres. From the above discussion, we can conclude that the formation of uniform La₂(MoO₄)₃ microspheres here is due to the self-aggregation and Ostwald ripening of nanoparticles.^32–34

Scheme 1 A possible formation mechanism of as-synthesized La₂(MoO₄)₃:Eu³⁺ particles.

3.3 Optical properties

La₂(MoO₄)₃ has been demonstrated to be a useful host for other lanthanide ions, which generate phosphors emitting in the UV-vis region.³⁵ Here, Eu³⁺ was selected as the doping ion to investigate the luminescence properties of the products with different amount of Na₃Cit. From Fig. 8, we can see that the emission spectra of the two samples are similar in shape, but different in the intensity to some extent. This emission spectrum was taken at an excitation wavelength of 294 nm, with the ⁵D₀–⁷F₂ transition centered at 615 nm being the most prominent group, which are composed of a series of ⁵D₀–⁷F_J (J = 1, 2, 3, 4) transition lines of Eu³⁺ as labeled in Fig. 8. The ⁵D₀–⁷F₂ electric dipole transition will be preponderant when Eu³⁺ ion occupied the lattice site of non centro-symmetric environment. If Eu³⁺ is located in a site with an inversion center, the ⁵D₀–⁷F₁ magnetic dipole transition should be dominant. In the present work, prominent red emission centered at 615 nm (⁵D₀–⁷F₂) was found to be much stronger than the orange emission at 591 nm (⁵D₀–⁷F₁) in both samples, indicating that Eu³⁺ ions effectively occupy the sites without inversion center.²⁸ As can be seen, the intensity of the peaks with 1 mmol Na₃Cit (a) was much stronger than that with 0 mmol Na₃Cit (b), indicating that the luminescence properties are closely correlated with amount of Na₃Cit introduced precursor solution. It is commonly accepted that the PL properties of inorganic materials are strongly dependent on their sizes, morphologies, and crystallinity.³⁶ Although the exact function of Na₃Cit on the spectral intensity in our synthetic process was not yet to be fully understood, it was believed that two properties of Na₃Cit play key roles in determining the spectral intensity. One is that Na₃Cit coordinates with La³⁺ ions in the solvent and form the complex compound La³⁺–Cit³⁻, then increase the effective light-absorbing cross-sectional area of the samples which result in the increase of the molar absorptivity. The other is that the morphology of products with Na₃Cit is uniform, ranged regularly and tightly.

Fig. 8 PL emission spectra for La₂(MoO₄)₃:Eu³⁺ prepared at 180 °C for 12 h with (a) and without (b) 1 mmol Na₃Cit.

To fully explore the luminescence properties of the as-obtained La₂(MoO₄)₃:Eu³⁺ sample, the photoluminescence (PL) and photoluminescence excitation (PLE) spectra, and the effects of reaction time on luminescence properties were investigated in detail. From Fig. 9, it can be clearly seen that the excitation and emission spectra at different reaction time (3 h, 6 h, 12 h, 24 h) of the as-prepared La₂(MoO₄)₃:Eu³⁺sample are similar in shape, but different in the intensity to some extent, indicating that the luminescence properties are closely correlated with the morphology, size and crystallinity of the products. Fig. 9a shows the PL excitation (PLE) spectra of the sample monitored at 615 nm of emission wavelength. The intense and broad absorption band within the wavelength range of 250–350 nm referred to as the charge transfer band (CTB). The main reason for the appearance of the CTB is the charge transfer from the filled 2p orbitals of the O²⁻ anion to the 4d orbitals of Mo⁶⁺ ions in MoO₄²⁻ groups of the host crystal. The second contribution to the CTB is due to the intervalence charge transfer between Mo and Eu.³⁷ It was also observed to be very weak bands in the long wavelength region which are attributed to the internal f–f transitions of Eu³⁺ ions.

Fig. 9 (a) Excitation spectra monitoring ⁵D₀ → ⁷F₂ fluorescence at 615 nm and (b) emission spectra for La₂(MoO₄)₃:Eu³⁺ prepared at 180 °C for different reaction stages: 3 h, 6 h, 12 h, 24 h.

Fig. 9b shows the PL emission spectrum of the as-prepared La₂(MoO₄)₃:Eu³⁺ sample. This emission spectrum was taken at an excitation wavelength of 294 nm and composed of a series of ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transition lines of Eu³⁺. The prominent peak at 615 nm is due to the hypersensitive electric dipole transition (⁵D₀ → ⁷F₂) of Eu³⁺ ions, which indicates that Eu³⁺ ions occupy sites without inversion symmetry.³⁸ The above energy transfer and emission process for MoO₄²⁻ and Eu³⁺ are schematically shown in Fig. 10.³⁹ It is noteworthy that the emission intensity of the sample after 12 h of hydrothermal treatment was higher than the samples obtained under different heating conditions. This result also shows that the as-synthesized La₂(MoO₄)₃:Eu³⁺ samples with the twinned hemispheres architectures are a kind of efficient phosphor with high red color purity, as well as the possible potential application. It is widely accepted that morphology, size and crystallinity are important factors influencing the luminescence properties of phosphors.

Fig. 10 Schematic diagram for the energy transfer from MoO₄²⁻ to Eu³⁺ and the emission process from Eu³⁺ ions.

4. Conclusions

In conclusion, La₂(MoO₄)₃:Eu³⁺ compound was successfully synthesized by the sodium citrate (Na₃Cit) assisted hydrothermal method and the FE-SEM images confirmed the formation of twinned hemispheres architectures. All the samples can be found in an agreement with the JCPDS card no. 45-0407. The dosage of sodium citrate, reaction temperature and reaction time all have an important influence on the structural and morphological evolution of La₂(MoO₄)₃:Eu³⁺phosphors. The PLE spectra of all the La₂(MoO₄)₃:Eu³⁺samples show the similar features and the effective energy absorption band was mainly located in the short ultraviolet region of 250–350 nm (λ _em = 615 nm). The PL spectra excited by a 294 nm wavelength demonstrated that the La₂(MoO₄)₃:Eu³⁺ exhibited a efficient red fluorescence, corresponding to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ion. The possible formation mechanisms for these unique morphologies had also been proposed on the basis of a series of time-dependent experiments. Due to their excellent PL intensity, high crystallinity and morphology uniformity, these phosphors have potential applications in photoelectric devices, such as UV-LED, FED, VFD devices, etc. This work will not only be helpful for the exploration and expansion of potential applications of La₂(MoO₄)₃ but also provide a new strategy to modify novel multifunctional nano/micro-structures.

Acknowledgements

The project was supported financially by the Training Fund of NENU's Scientific Innovation Project (NENU-STC07014).

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